U.S. patent number 6,856,284 [Application Number 10/689,626] was granted by the patent office on 2005-02-15 for methods and apparatus for multi-beam, multi-signal transmission for active phased array antenna.
This patent grant is currently assigned to ITT Manufacturing Enterprises, Inc.. Invention is credited to Gene L. Cangiani.
United States Patent |
6,856,284 |
Cangiani |
February 15, 2005 |
Methods and apparatus for multi-beam, multi-signal transmission for
active phased array antenna
Abstract
A method and apparatus for simultaneously transmitting, at a
common frequency, a plurality of signals on a plurality of beams
from a phased array antenna involves forming composite signals that
contain the plurality of signals. The composite signals correspond
to respective antenna elements in the antenna array and include at
least a first signal for transmission via a first transmit beam and
a second signal for transmission via a second transmit beam. The
phases of the composite signals are a function of signal
modulations of the first and second signals and phases of the
respective antenna elements required to form the first and second
transmit beams. By digitally forming the composite signals, taking
into account the modulation phase and beam-forming phase of each
signal at each antenna element, hardware requirements are reduced
and efficient, saturated amplifier can be employed.
Inventors: |
Cangiani; Gene L. (Parsippany,
NJ) |
Assignee: |
ITT Manufacturing Enterprises,
Inc. (Wilmington, DE)
|
Family
ID: |
34116832 |
Appl.
No.: |
10/689,626 |
Filed: |
October 22, 2003 |
Current U.S.
Class: |
342/372; 342/154;
342/373 |
Current CPC
Class: |
H01Q
1/288 (20130101); H01Q 3/34 (20130101); H04B
7/0617 (20130101); H01Q 25/00 (20130101); H01Q
25/002 (20130101); H01Q 21/06 (20130101) |
Current International
Class: |
H01Q
1/28 (20060101); H01Q 1/27 (20060101); H01Q
3/34 (20060101); H01Q 3/30 (20060101); H01Q
21/06 (20060101); H01Q 25/00 (20060101); H04B
7/04 (20060101); H04B 7/06 (20060101); H01Q
003/24 () |
Field of
Search: |
;342/81,154,368,372,373 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Phan; Dao
Attorney, Agent or Firm: Edell, Shapiro & Finnan,
LLC
Claims
What is claimed is:
1. A method of transmitting a plurality of signals from a common
antenna, comprising: (a) generating a first signal for transmission
via a first transmit beam; (b) generating a second signal for
transmission via a second transmit beam; (c) forming a composite
signal that includes the first and second signals, wherein the
phase of the composite signal accounts for signal modulation and
beam forming characteristics of the first and second signals, and
wherein the composite signal has a constant amplitude envelope; (d)
supplying the composite signal to the common antenna; and (e)
transmitting the composite signal from the common antenna, thereby
transmitting the first signal via the first transmit beam and
transmitting the second signal via the second transmit beam.
2. The method of claim 1, wherein the first signal is different
from the second signal.
3. The method of claim 1, wherein the first and second signals are
transmitted at a common frequency.
4. The method of claim 1, wherein the first transmit beam is a
Global Positioning System (GPS) earth coverage beam and the second
transmit beam is a steerable GPS spot beam.
5. The method of claim 1, wherein the common antenna is a phased
array antenna comprising an array of antenna elements, and wherein:
(c) includes forming a plurality of composite signals, wherein
phases of the composite signals are a function of phases of
respective antenna elements required to form the first and second
transmit beams; and (d) includes supplying the plurality of
composite signals to the respective antenna elements of the phased
array antenna.
6. The method of claim 5, wherein at least one of the first and
second transmit beams is electronically steerable by adjusting
phases of the composite signals.
7. A method of transmitting a plurality of signals from a common
phased array antenna comprising an array of antenna elements, the
method comprising: (a) generating a plurality of first modulated
signals corresponding to antenna elements for transmission via a
first transmit beam, and phase shifting the first modulated signals
in accordance with phases of the antenna elements required to form
the first transmit beam; (b) generating, separate from the
plurality of first modulated signals, a plurality of second
modulated signals corresponding to antenna elements for
transmission via a second transmit beam, and phase shifting the
second modulated signals in accordance with phases of the antenna
elements required to form the second transmit beam; (c) forming a
Plurality of composite signals by combining phase shifted first
modulated signals with respective phase shifted second modulated
signals, wherein phases of the composite signals are a function of
signal modulations of the first and second modulated signals and
phases of the respective antenna elements required to form the
first and second transmit beams: (d) supplying the plurality of
composite signals to the respective antenna elements of the phased
array antenna; and (e) transmitting the composite signal from the
common antenna, thereby transmitting the first modulated signal via
the first transmit beam and transmitting the second modulated
signal via the second transmit beam.
8. The method of claim 7, wherein (a) includes attenuating the
phase shifted first modulated signals and (b) includes attenuating
the phase shifted second modulated signals to control a
distribution of power between the first and second transmit
beams.
9. The method of claim 5, wherein (c) includes: forming a plurality
of digital composite signals corresponding to the respective
antenna elements, wherein phases of the digital composite signals
are a function of modulation of the first and second signals and
phases of the respective antenna elements required to form the
first and second transmit beams; converting the digital composite
signals to analog composite signals; modulating carrier signals
with the analog composite signals to form the plurality of
composite signals.
10. The method of claim 9, wherein: (a) includes determining,
phases of a plurality of first digital signals corresponding to the
respective antenna elements, based on a modulation of the first
signal and phases of the respective antenna elements required to
form the first transmit beam; (b) includes determining phases of a
plurality of second digital signals corresponding to the respective
antenna elements, based on a modulation of the second signal and
phases of the respective antenna elements required to form the
second transmit beam; and (c) includes forming the plurality of
digital composite signals based on the phases and amplitudes of the
plurality of first and second digital signals.
11. The method of claim 10, wherein the digital composite signals
are computed as a sum of the first and second digital signals.
12. The method of claim 10, wherein the digital composite signals
are formed by interleaving the first and second digital
signals.
13. The method of claim 9, further comprising: (f) amplifying the
plurality of composite signals using respective saturated high
power amplifiers.
14. An apparatus for transmitting a plurality of signals,
comprising: a phased array antenna comprising an array of antenna
elements; and a transmitter system that receives a first signal for
transmission via a first transmit beam and a second signal for
transmission via a second transmit beam, the transmitter system
forming a plurality of composite signals and supplying the
plurality of composite signals to respective antenna elements of
the phased array antenna, wherein phases of the composite signals
are a function of signal modulations of the first and second
signals and phases of the respective antenna elements required to
form the first and second transmit beams, and wherein each of the
composite signals has a constant amplitude envelope; wherein the
phased array antenna transmits the first signal via the first
transmit beam and transmits the second signal via the second
transmit beam.
15. The apparatus of claim 14, wherein the first signal is
different from the second signal.
16. The apparatus of claim 14, wherein the phased array antenna
transmits the first and second signals at a common frequency.
17. The apparatus of claim 14, wherein at least one of the first
and second transmit beams is electronically steerable by adjusting
phases of the composite signals.
18. The apparatus of claim 14, wherein: the apparatus is a Global
Positioning System (GPS) satellite, and wherein the first transmit
beam is a GPS earth coverage beam and the second transmit beam is a
steerable GPS spot beam.
19. An apparatus for transmitting a plurality of signals,
comprising: a phased array antenna comprising an array of antenna
elements; and a transmitter system that receives a first signal for
transmission via a first transmit beam and a second signal for
transmission via a second transmit beam, the transmitter system
forming a plurality of composite signals and supplying the
plurality of composite signals to respective antenna elements of
the phased array antenna, wherein phases of the composite signals
are a function of signal modulations of the first and second
signals and phases of the respective antenna elements required to
form the first and second transmit beams; wherein the phased array
antenna transmits the first signal via the first transmit beam and
transmits the second signal via the second transmit beam; and
wherein the transmitter system further comprises: a first power
splitter that produces a plurality of first modulated signals from
the first signal; a first array of phase shifters that phase shift
the first modulated signals in accordance with phases of the
respective antenna elements required to form the first transmit
beam; a second power splitter that produces a plurality of second
modulated signals from the second signal; a second array of phase
shifters that phase shift the second modulated signals in
accordance with phases of the respective antenna elements required
to form the second transmit beam; and a combiner that combines the
phase shifted first modulated signals with respective phase shifted
second modulated signals to form the plurality of composite
signals.
20. The apparatus of claim 19, further comprising a first array of
attenuators that attenuate the first modulated signals and a second
array of attenuators that attenuate the second modulated signals to
control a distribution of power between the first and second
transmit beams.
21. The apparatus of claim 19, further comprising an array of
linear amplifiers that respectively amplify the plurality of
composite signals.
22. The apparatus of claim 14, wherein the transmitter system
further comprises: a processor that forms a plurality of digital
composite signals corresponding to the respective antenna elements,
wherein phases of the digital composite signals are a function of
modulation of the first and second signals and phases of the
respective antenna elements required to form the first and second
transmit beams; an array of digital-to-analog converters that
convert the digital composite signals to analog composite signals;
and an array of signal modulators that modulate carrier signals
with the analog composite signals to form the plurality of
composite signals.
23. The apparatus of claim 22, wherein the processor: determines
phases of a plurality of first digital signals corresponding to the
respective antenna elements based on a modulation of the first
signal and phases of the respective antenna elements required to
form the first transmit beam; determines phases of a plurality of
second digital signals corresponding to the respective antenna
elements based on a modulation of the second signal and phases of
the respective antenna elements required to form the second
transmit beam; and forms the plurality of digital composite signals
based on the phases and amplitudes of the plurality of first and
second digital signals.
24. The apparatus of claim 23, wherein the processor computes the
digital composite signals as a sum of the first and second digital
signals.
25. The apparatus of claim 23, wherein the processor forms the
digital composite signals by interleaving the first and second
digital signals.
26. The apparatus of claim 22, further comprising: an array of
saturated high power amplifiers that respectively amplify the
plurality of composite signals.
27. An apparatus for transmitting a plurality of signals,
comprising: a phased array antenna comprising an array of antenna
elements; and means for forming a plurality of composite signals
from a first signal and a second signal, and for supplying the
plurality of composite signals to respective antenna elements of
the phased array antenna for transmission, wherein phases of the
composite signals are a function of signal modulations of the first
and second signals and phases of the respective antenna elements
required to transmit the first signal via a first transmit beam and
to transmit the second signal via a second transmit beam, and
wherein each of the composite signals has a constant amplitude
envelope.
28. A method of transmitting a plurality of signals from a common
antenna, comprising: (a) generating a first signal for transmission
via a first transmit beam; (b) generating a second signal for
transmission via a second transmit beam; (c) forming a composite
signal by time interleaving the first signal and the second signal,
wherein the phase of the composite signal accounts for signal
modulation and beam forming characteristics of the first signal
and, alternately, the signal modulation and beam forming
characteristics of the second signal; (d) supplying the composite
signal to the common antenna; and (e) transmitting the composite
signal from the common antenna, thereby transmitting the first
signal via the first transmit beam and transmitting the second
signal via the second transmit beam.
29. The method of claim 28, wherein the common antenna is a phased
array antenna comprising an array of antenna elements, and wherein:
(c) includes forming a plurality of composite signals, wherein
phases of the composite signals are a function of phases of
respective antenna elements required to form the first and second
transmit beams; and (d) includes supplying the plurality of
composite signals to the respective antenna elements of the phased
array antenna.
30. The method of claim 29, wherein (c) includes: forming a
plurality of digital composite signals corresponding to the
respective antenna elements, wherein phases of the digital
composite signals are a function of modulation of the first and
second signals and phases of the respective antenna elements
required to form the first and second transmit beams; converting
the digital composite signals to analog composite signals;
modulating carrier signals with the analog composite signals to
form the plurality of composite signals.
31. The method of claim 30, wherein: (a) includes determining
phases of a plurality of first digital signals corresponding to the
respective antenna elements, based on a modulation of the first
signal and phases of the respective antenna elements required to
form the first transmit beam; (b) includes determining phases of a
plurality of second digital signals corresponding to the respective
antenna elements, based on a modulation of the second signal and
phases of the respective antenna elements required to form the
second transmit beam; and (c) includes forming the plurality of
digital composite signals based on the phases and amplitudes of the
plurality of first and second digital signals.
32. An apparatus for transmitting a plurality of signals,
comprising: a phased array antenna comprising an array of antenna
elements; and a transmitter system that receives a first signal for
transmission via a first transmit beam and a second signal for
transmission via a second transmit beam, the transmitter system
forming a plurality of composite signals by time interleaving the
first signal and the second signal, and supplying the plurality of
composite signals to respective antenna elements of the phased
array antenna, wherein phases of the composite signals are a
function of a signal modulation of the first signal and phases of
the respective antenna elements required to form the first transmit
beam and, alternately, a function of a signal modulation of the
second signal and phases of the respective antenna elements
required to form the second transmit beam; wherein the phased array
antenna transmits the first signal via the first transmit beam and
transmits the second signal via the second transmit beam.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus for
generating multi-beam, multi-signal transmissions using an active
phased array antenna.
2. Description of the Related Art
There are many communications, radar, and navigation applications
that require multiple transmit beams from a common site, with
different signals on the various beams. For example, the next
generation of Global Positioning System (GPS) satellites will
require the transmission of spot beams to direct higher power
military signals to theaters of operation, along with broader earth
coverage beams for conventional navigational users. Proposed
solutions include the use of a separate gimbaled antenna to provide
the spot beam, in addition to the existing earth coverage antenna.
Alternative approaches using an active phased array antenna with an
electronically steered beam to implement the spot beam have also
been proposed.
The use of a separate antenna (mechanically steered or
electronically steered) for each beam also requires the use of a
separate transmitter and associated electronics to drive each
antenna. This greatly increases system cost, size, weight and power
requirements over what would be required for a single phased array
antenna and transmitter. For example, separate High Power
Amplifiers (HPAs) would be required to drive each antenna. Also,
the use of separate antennas and separate transmitters for the
various beams (e.g., earth coverage and spot beams) limits the
ability to rc-allocate power among the various beams. This
flexibility would provide significant benefit for many
applications. For example, the ability to re-allocate power among
beams would provide more power to support additional missions with
the earth coverage beam when the spot beams are not in use--a very
desirable feature for future versions of GPS.
Note that active phased array antennas have been configured to
provide multiple simultaneous beams with the same signal on all of
the beams. However, a phased array configuration with different
signals on each of the beams has not previously been implemented.
Preferably, a system capable of producing multiple beams for
multiple signals would permit the use of composite signals whose
amplitude envelopes are constant. If non-constant envelope signals
are applied to the HPAs, the use of highly efficient, saturated
HPAs is precluded. Linear methods that generate
non-constant-envelope composite signals result in power-inefficient
mechanizations, because the power amplifiers that are used for
transmission of the composite signals must operate in the linear
region. Power amplifiers are much more efficient when operated in
the saturated mode. For example, linear superposition of
chip-synchronous, orthogonal signals to be transmitted is a
theoretically lossless multiplex if the subsequent transmission
chain remains linear. Maintaining linearity requires a linear high
power amplifier (HPA). Since any HPA characteristic eventually
saturates as its input power increases, such base station
transceiver linear amplifiers are typically run at 4-5 dB average
power backoff to accommodate peak power needs.
Thus, linear combination techniques are maximally efficient in the
sense that there is no actual signal power loss, but the overall
efficiency of such techniques is compromised by the need to operate
the amplifier at a significant power back-off to accommodate the
instantaneous signal envelope fluctuations. An alternative approach
to producing greater average power is to achieve a more effective
allocation of the loss budget between the multiplexer and the high
power amplifier. Non-linear multiplex methods that produce a
constant-envelope composite signal permit a greater fraction of the
available transmitter power to be used for communication, but at
the expense of a multiplexing loss that may be characterized as an
intermodulation product. This multiplexing loss, however, is
typically smaller than the power backoff it replaces, resulting in
a favorable trade. Therefore, constant-envelope signal structures
are required if full-power, undistorted transmission is sought.
Consequently, in developing a scheme to simultaneously transmit
multiple beams with multiple signals, it would be desirable to
transmit from each antenna element in the array a constant
amplitude envelope composite signal to permit the use of
power-efficient, saturated HPAs.
SUMMARY OF THE INVENTION
Therefore, in light of the above, and for other reasons that become
apparent when the invention is fully described, an object of the
present invention is to simultaneously transmit a plurality of
different signals on a plurality of different beams at the same
carrier frequency from a common antenna.
A further object of the present invention is to reduce the overall
hardware for a system required to transmit multiple different
signals on multiple different beams from a common site or location,
thereby reducing system cost, weight, size and power.
Yet a further object of the present invention is to dynamically
allocate power among a plurality of beams being simultaneously
transmitted by a common antenna.
Another object of the present invention is to efficiently generate
constant-envelope signals to allow use of saturated high power
amplifiers in signal transmission.
The aforesaid objects are achieved individually and in combination,
and it is not intended that the present invention be construed as
requiring two or more of the objects to be combined unless
expressly required by the claims attached hereto.
In accordance with the present invention, a plurality of signals
can be simultaneously transmitted on a plurality of beams from a
phased array antenna at a common frequency. The technique involves
forming composite signals that correspond to respective antenna
elements in the antenna array. The phases of the composite signals
are a function of signal modulations of the component signals and
phases of the respective antenna elements required to form the
corresponding beams. By digitally forming the composite signals,
taking into account the modulation phase and beam-forming phase of
each signal at each antenna element, multiple individually
steerable beams, each with it's own unique signal, can be
generated.
More specifically, for example, a method of transmitting two
signals from a common antenna includes: generating a first signal
for transmission via a first transmit beam; generating a second
signal for transmission via a second transmit beam; forming a
composite signal that includes the first and second signals,
wherein the phase of the composite signal accounts for signal
modulation and beam forming characteristics of the first and second
signals; and transmitting the composite signal from the common
antenna, thereby transmitting the first signal via the first
transmit beam and transmitting the second signal via the second
transmit beam. For example, the first transmit beam can be a GPS
earth coverage beam and the second transmit beam can be a steerable
GPS spot beam. This can be easily extended to include additional
beams with additional signals.
The common antenna can be a phased array antenna comprising an
array of antenna elements, where a plurality of composite signals
corresponding to respective antenna elements are formed, with the
phases of the composite signals being a function of the signal
modulation and the phases of respective antenna elements required
to form the plurality of transmit beams.
In accordance with one embodiment, a plurality of first modulated
signals are phase shifted in accordance with phases of the
respective antenna elements required to form the first transmit
beam, and a plurality of second modulated signals are separately
phase shifted in accordance with phases of the respective antenna
elements required to form the second transmit beam. The two sets of
phase shifted signals are then combined to form the plurality of
composite signals. The first and second modulated signals can be
separately attenuated to control the distribution of power between
the first and second transmit beams.
In accordance another embodiment, a plurality of digital composite
signals corresponding to the respective antenna elements are
formed, wherein phases of the digital composite signals are a
function of modulation of the first and second signals and phases
of the respective antenna elements required to form the first and
second transmit beams. The digital composite signals are converted
to analog composite signals and carrier signals are modulated with
the analog composite signals to form the plurality of composite
signals. Phases of a plurality of first digital signals
corresponding to the respective antenna elements are determined
based on a modulation of the first signal and phases of the
respective antenna elements required to form the first transmit
beam. Similarly, phases of a plurality of second digital signals
corresponding to the respective antenna elements are determined
based on a modulation of the second signal and phases of the
respective antenna elements required to form the second transmit
beam. The plurality of digital composite signals are then formed
based on the phases and amplitudes of the plurality of first and
second digital signals.
With this embodiment, according to one approach, the digital
composite signals are computed from a vector sum of the first and
second digital signals. More specifically, for each antenna
element, a composite phase is determined based on the phase and
amplitude of the component signals, where the phase of each
component signal is, in turn, determined from the instantaneous
modulation phase and beam forming phase of the component signal for
each antenna element. Advantageously, the composite signal can have
a constant envelope, which permits the use of efficient, saturated
high power amplifiers.
According to another approach, the digital composite signals are
formed by interleaving the first and second digital signals in a
time division manner. This approach avoids signal clipping that may
occur with summing the signals. Distribution of power between the
beams can be controlled by selecting the ratio of the time segments
allocated to each of the signals in the interleave pattern.
An apparatus for transmitting a plurality of signals in accordance
with the invention includes the aforementioned phased array antenna
and a transmitter system. The transmitter system receives a first
signal for transmission via a first transmit beam and a second
signal for transmission via a second transmit beam and possibly
additional signals for transmission via additional transmit beams,
and forms a plurality of composite signals for respective antenna
elements of the phased array antenna, wherein phases of the
composite signals are a function of signal modulations of the
multiple signals and phases of the respective antenna elements
required to form the multiple transmit beams.
The following descriptions of various embodiments assume that only
two signals are to be transmitted via two transmit beams, for
simplicity. Note that the invention is not limited to two signals
via two beams. It can be utilized to transmit several independent
signals, each via its own dedicated, independently steerable
beam.
According to the embodiment in which the first and second signals
are separately modulated and phase shifted, the transmitter system
includes a first power splitter that produces a plurality of first
modulated signals from the first signal; a first array of phase
shifters that phase shift the first modulated signals in accordance
with phases of the respective antenna elements required to form the
first transmit beam; a second power splitter that produces a
plurality of second modulated signals from the second signal; a
second array of phase shifters that phase shift the second
modulated signals in accordance with phases of the respective
antenna elements required to form the second transmit beam; and a
combiner that combines the phase shifted first modulated signals
with respective phase shifted second modulated signals to form the
plurality of composite signals. A first array of attenuators can be
used to attenuate the first modulated signals and a second array of
attenuators can be used to attenuate the second modulated signals
to control a distribution of power between the first and second
transmit beams. Since the composite signal does not have a constant
envelope in the embodiment, the transmitter system requires an
array of linear amplifiers that respectively amplify the plurality
of composite signals.
In accordance with the digital implementation, the transmitter
system includes a processor that forms a plurality of digital
composite signals corresponding to the respective antenna elements,
wherein phases of the digital composite signals are a function of
modulation of the first and second signals and phases of the
respective antenna elements required to form the first and second
transmit beams. An array of digital-to-analog converters convert
the digital composite signals to analog composite signals, and an
array of signal modulators modulate carrier signals with the analog
composite signals to form the plurality of composite signals.
The processor determines phases of a plurality of first digital
signals corresponding to the respective antenna elements based on a
modulation of the first signal and phases of the respective antenna
elements required to form the first transmit beam, and determines
phases of a plurality of second digital signals corresponding to
the respective antenna elements based on a modulation of the second
signal and phases of the respective antenna elements required to
form the second transmit beam. The processor then forms the
plurality of digital composite signals based on the phases and
amplitudes of the plurality of first and second digital signals.
This can be accomplished using the aforementioned summing approach
or the interleaving approach.
The above and still further objects, features and advantages of the
present invention will become apparent upon consideration of the
following definitions, descriptions and descriptive figures of
specific embodiments thereof wherein like reference numerals in the
various figures are utilized to designate like components. While
these descriptions go into specific details of the invention, it
should be understood that variations may and do exist and would be
apparent to those skilled in the art based on the descriptions
herein.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram illustrating a typical
two-antenna approach to generating plural beams for transmitting
plural signals.
FIG. 2 is a functional block diagram illustrating a transmitter
system for simultaneously transmitting plural signals on plural
beams from a common antenna in accordance with an exemplary
embodiment of the present invention.
FIG. 3 is a graph illustrating a convention for describing
vectors.
FIG. 4 is a graph illustrating vector addition of signals to
generate a constant-envelope composite signal.
FIG. 5 is a functional block diagram illustrating a transmitter
system for simultaneously transmitting plural signals on plural
beams from a common antenna in accordance with another embodiment
of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The system described herein provides a unique method for
simultaneously transmitting multiple beams, each with its own
signal component, from the same active phased array antenna. This
configuration eliminates the need for separate antennas and
transmitters for each of the required beams. The system is capable
of forming multiple simultaneous beams, for example an earth
coverage beam and multiple spot beams covering multiple theaters of
operation, with the capability to re-allocate power among the
various beams, which, in the context of satellite systems, can be
performed on-orbit. The composite signal applied to each high-power
amplifier (HPA) preferably, although not necessarily, has a
constant amplitude envelope, which permits the use of saturated
HPAs with their attendant power efficiency.
As used herein, the terms antenna beam(s), transmit beam(s) or
simply "beam(s)", including spot beams and earth-coverage beams,
refer generally to radiated energy concentrated in a certain
direction. The radiated antenna beam is formed by a directional
antenna and radiates over a particular angular region in accordance
with the antenna pattern (i.e., the radiated field intensity as a
function of angle), resulting in transmission of a directed signal.
The antenna beam may be fixed in a particular direction or may be
electronically or mechanically steered over a range of directions.
In the case of an antenna comprising an array of antenna elements
that individually transmit signals in a coordinated manner, it is
possible to electronically steer the antenna beam by controlling
the relative phases of the signals transmitted by the antenna
elements.
The term "multi-beam, multi-signal" refers to the simultaneous
transmission of two or more different beams (e.g., having different
directions and/or shapes) from a common antenna at the same carrier
frequency, wherein two or more different signals (e.g., containing
different information, having different timing, signal structure,
coding, etc.) are being transmitted via the beams, and at least one
of the beams carries a signal that is different from a signal
carried by at least one other of the beams. The simplest example is
a two-beam configuration (e.g., a GPS spot beam and earth coverage
beam) in which one of the beams carries a first signal and the
other beam carries a second, different signal, wherein both signals
are transmitted at the same frequency (e.g., the GPS L1 frequency).
The term "multi-beam, multi-signal" does not preclude more complex
arrangements (e.g., three or more beams wherein each beam can carry
its own unique signal or the same signal that is carried on one or
more of the other beams.)
To better appreciate the invention, a more typical two-antenna
configuration for generating two separate beams with separate
signals is first described. Referring to FIG. 1, a GPS satellite
transmitter system 10 employing two separate antennas is shown. The
GPS satellite is required to transmit signals at the GPS L-band
frequencies (L1 at 1575.42 MHz and L2 at 1227.6 MHz) via both
narrow beamwidth spot beams (e.g., for potential military
applications) and wider beamwidth earth-coverage beams (e.g., for
civilian and commercial use). To meet these requirements, system 10
includes a first, spot-beam antenna 12 and a second,
earth-coverage-beam antenna 14. Antennas 12 and 14, represented in
FIG. 1 by ovals, can be phased-array antennas comprising arrays of
antenna elements (the multi-element nature of the antennas is
suggested by the smaller ovals depicted within the larger antenna
ovals). Because beamwidth is inversely proportional to antenna
size, spot-beam antenna 12 is physically larger than earth-coverage
antenna 14, as suggested by the relative oval sizes in FIG. 1. The
size of the antennas is determined by the smallest spot or
footprint required. The smaller, earth-coverage-beam antenna 14,
for example, can be approximately three feet in diameter if beam
shaping is required, or can be somewhat smaller with no beam
shaping. To enable transmission at both the L1 and L2 frequencies,
both antennas 12 and 14 include antenna elements that transmit at
the L1 frequency interspersed with antenna elements that transmit
at the L2 frequency. This configuration is really four antennas
occupying the physical space of two antennas, via interleaving of
the elements.
Referring to the spot beam transmitter system shown on the left
side of FIG. 1, a spot beam signal at the L1 frequency is amplified
by a preamplifier 16 and supplied to a power splitter 18 that
distributes the L1 spot beam signal to an array of variable phase
shifters 20 corresponding to respective L1 antenna elements in the
spot beam antenna array. Each of the variable phase shifters 20
imparts a phase shift on the input L1 spot-beam signal in
accordance with an individual phase command. The set of phase
commands respectively supplied to the array of variable phase
shifters 20 causes the spot beam to be electronically steered in a
particular direction. By adjusting the relative phases of the phase
commands, the beam can be steered over a range of angles as needed
to support particular GPS functions or operations employing spot
beams. An array of high-power amplifiers (HPAs) 22 corresponding to
respective L1 antenna elements in the antenna array amplifies the
phase-shifted L1 spot beam signals from the variable phase shifters
20 prior to transmission of the signals by the respective L1
antenna elements.
Similarly, a spot beam signal at the L2 frequency is amplified by a
preamplifier 24 and supplied to a power splitter 26 that
distributes the L2 spot beam signal to an array of variable phase
shifters 28 that correspond to respective L2 antenna elements in
the antenna array and effect beam steering of the L2 spot beam.
Each of the L2 spot beam signals is amplified by a corresponding
HPA 30 (one HPA for each L2 antenna element) and transmitted via a
corresponding L2 antenna element, such that the array of
phase-shifted L2 spot beam signals forms the electronically steered
L2 spot beam.
In the system shown in FIG. 1, the L1 and L2 earth-coverage beams
are generated using a separate earth coverage antenna requiring
additional hardware to generate the L1 and L2 earth coverage beams.
Specifically, referring to the earth-coverage beam transmitter
system shown on the right side of FIG. 1, an earth coverage beam
signal at the L1 frequency is amplified by a preamplifier 32 and
supplied to a power splitter 34 that distributes the L1 spot beam
signal to an array of fixed phase shifters 36. Because the earth
coverage beam is not electronically steered, fixed phase shifters
can be used. Due to symmetry, certain L1 antenna elements will have
the same phase shift, allowing sets of elements with the same fixed
phase shift to be grouped. Consequently, a single fixed phase
shifter and HPA can be used to drive a set of antenna elements
having the same phase shift. After amplification by the HPAs 38,
the L1 earth coverage beam signals are transmitted via the L1 earth
coverage antenna elements to form the earth coverage beam.
Similarly, an earth coverage beam signal at the L2 frequency is
amplified by a preamplifier 40 and supplied to a power splitter 42
that distributes the L2 earth coverage beam signal to an array of
phase shifters 44. Each of the phase-shifted signals is amplified
by a corresponding HPA 46 prior to transmission. As with the L1
earth coverage beam, because sets of L2 earth coverage beam antenna
elements have the same phase shift, certain elements can be grouped
and driven by a single fixed phase shifter and HPA, such that the
number of fixed phase shifters and HPAs may be less than the number
of L2 antenna elements. As will be appreciated from the
configuration shown in FIG. 1, two separate antennas are required
to generate the spot beam and the earth coverage beam, and each of
these antennas requires the complete transmitter hardware for
generating signals at the L1 and L2 frequencies (i.e., two antennas
and four sets of transmitter hardware). These hardware requirements
make this configuration costly. Moreover, there is no possibility
of dynamically allocating power between the spot beam and
earth-coverage beam to adapt to varying operational requirements or
conditions.
The signal combining techniques of the present invention permit
multiple, different antenna beams carrying multiple, different
signals to be simultaneously transmitted using a single set of
transmitter hardware and a single antenna. More specifically, the
combining technique operates on a particular RF carrier to
simultaneously transmit via a common antenna (e.g., a phased-array
antenna) at least first and second signals at the same frequency,
wherein the first signal is transmitted via a first antenna beam,
and the second signal is transmitted via a second antenna beam that
can, in general, be different from the first antenna beam in
direction, beamwidth, power, antenna gain pattern, etc.
Most signals of interest for communication and navigation
applications are different phase modulations of the same RF
carrier, and beam-forming is accomplished by imparting additional
phase shifts to the signals that are applied to each of the antenna
elements. An important concept underlying the present invention is
to combine phase modulation and beam-forming to derive a composite
phase for application to each array element. At any instant of
time, the signal phase will vary from one array element to another
in order to effect the configuration of beams, while, at any one
array element the signal phase varies in time due to the phase
modulation on the various signals. A variety of methods can be used
to combine the various signal components to achieve the desired
composite signal, and the optimal combining technique is dependent
on the specific application. Some example techniques are described
herein.
The most straight-forward signal combining technique to achieve the
desired result of multiple beams with corresponding different
signals is to sum the signals corresponding to each beam. FIG. 2
illustrates a signal combining system 50 that implements this
approach in the context of a GPS satellite transmitter required to
transmit both a spot beam and an earth coverage beam at a
particular frequency. The components of the signal combining system
50 shown in FIG. 2 relate to a single antenna element of a
phased-array antenna (or a group of antenna elements where certain
antenna elements always have the same phase due to symmetry). The
overall transmitter system includes a plurality of these
components, with one set of components for each antenna element (or
group of elements) in the antenna array.
For each antenna element (or group), signal combining system 50
includes a phase shifter 52 that phase shifts the earth coverage
signal (designated as the "EC Signal") at a particular frequency
and phase shifter 54 that phase shifts the spot beam signal at the
same frequency. The input earth coverage signal shown in FIG. 2 is
one of an array of such signals generated by splitting an initial
earth coverage signal via a power splitter (not shown) after
amplification by a pre-amplifier (not shown) and corresponding to
the array of antenna elements. The input spot beam signal shown in
FIG. 2 is likewise one of an array of such signals generated in a
similar manner. If the direction of the earth coverage beam remains
constant over time, phase shifter 52 can be a fixed phase shifter.
Phase shifter 54 is preferably a digitally controlled variable
phase shifter to permit electronic steering of the spot beam.
The phase-shifted earth coverage signal and spot beam signal are
respectively attenuated by attenuators 56 and 58, which are used to
allocate power between the two beams. The attenuated earth coverage
and spot beam signals, which are both constant envelope signals,
are supplied to a zero phase hybrid 60, which can be, for example,
a standard RF signal combiner such as a Wilkinson Combiner. The
resulting composite signal has a non-constant envelope.
Consequently, a linear amplifier 62 is used to amplify the
composite signal prior to transmission by an antenna element 64 (or
group of elements where permitted by symmetry).
This configuration produces a dual beam (earth coverage and spot)
with separate signals on each beam, and provides the capability to
allocate total power between the two beams by adjusting the
attenuators. One disadvantage of this technique is that the
composite signal that is supplied to the amplifier is not a
constant envelope signal, so the amplifier needs to be a linear
amplifier, which is less efficient than a saturated amplifier, as
described above. Also, if additional beams with additional signals
are appended, additional headroom must be allocated in the linear
amplifier to accommodate larger amplitude swings in the composite
signal, further degrading power efficiency.
In accordance with another embodiment, power efficiency can
generally be improved by adding a third signal component that
results in a constant envelope composite signal. At any instant of
time, the earth coverage signal and the spot beam signal will have
an arbitrary phase shift between them, since the absolute phase of
each signal is the sum of the phase from its modulating signal and
the phase required to form its desired beam shape. In the context
of the GPS example involving a spot beam and earth coverage beam,
the beam forming phase angle for the spot beam is generally much
slower changing than the phase angle from the signal modulation,
and the phase angle for the fixed earth coverage beam is constant
for each antenna element.
Understanding of the concept of generating a constant envelope
signal using an additional signal component is facilitated by the
vector analysis described in conjunction with FIGS. 3 and 4.
Referring to FIG. 3, a vector a can be described as consisting of a
magnitude, a, and the angle, .theta., that it forms with the
in-phase axis, I, a=a<.theta., wherein the character < means
"at angle." Then, the magnitudes of the components of a along the I
and Q axes are: a.sub.i =a*cos(.theta.) and a.sub.q
=a*sin(.theta.), respectively.
Referring now to FIG. 4, let the spot beam signal be represented as
the vector s=s<.alpha., and the earth coverage signal as the
vector e=e<.beta.. The circle shown in FIG. 4 represents a
constant envelope. The circle has a radius equal to the maximum
possible amplitude of the sum of the spot beam signal and the
earth-coverage-beam signal, which results when the spot beam signal
vector and the earth-coverage-beam signal vector are at the same
angle (i.e., the scalar sum of the amplitudes of the two beams). If
at some instant of time, these vectors are collinear (e.g., the
dashed orientation for e shown in FIG. 4), the composite signal
magnitude, c, would be e+s, or the radius of the circle.
Consequently, once the individual levels of the spot-beam signal
and earth-coverage beam signals have been determined, the composite
amplitude, c, of the constant envelope signal is set to the sum of
those two signal levels, or c=e+s.
At any arbitrary instant of time, the composite signal can be
derived as follows:
Then the angle, .theta., of the composite signal can be
determined:
The in-phase (I) and quadrature (Q) channel inputs can then be
computed as:
As shown in FIG. 4, when the two vectors are not at same angle, the
amplitude of the resultant vector e+s is less than the radius of
the circle. By adding a third signal component at angle .theta.,
the magnitude of resultant vector can be extended out to the
constant envelope circle. Thus, the desired composite signal can be
constructed as the vector at angle .theta. with its magnitude
scaled to the desired magnitude, c=e+s (circle radius). Then, at
every instant of time, the computed composite signal will have the
same amplitude, allowing the use of highly efficient, saturated
amplifiers. The in-phase and quadrature channel inputs can thus be
easily calculated for each antenna array element.
The resultant composite signal can be viewed as the vector
summation of the spot beam signal vector and the earth coverage
signal vector, plus the additional third signal component that
serves to re-scale the signal to the appropriate amplitude. The net
result of adding this third signal component is the composite
signal, c, which has a constant amplitude (radius of the circle),
allowing the use of highly efficient saturated amplifiers. This
concept can be easily extended to more than two beams with a unique
signal on each beam, while maintaining a constant amplitude
(envelope) signal. The power contained in the additional signal
component, labeled "Third Signal Component" in FIG. 4, is wasted
power. It is the "cost" of obtaining a constant envelope composite
signal, similar to the intermodulation component in an interplex
modulator. As with interplex modulation, overall efficiency is
significantly improved over the configuration with linear
amplifiers. The disadvantage of this technique is that it is
equivalent to "clipping" the desired composite signal vector and,
depending upon the application, it may not be possible to exercise
effective control over the relative power ratios among the various
signal components.
Note that it is not necessary to implement three signal channels
for each array element in order to achieve this constant envelope
condition. The composite signal vector c in FIG. 4 can be
calculated, and then the in-phase and quadrature components can be
calculated as c*cos(.theta.) and c*sin(.theta.), respectively,
where the constant magnitude c is equal to the sum of the vector
magnitudes e+s. Then these two components can be applied to
in-phase and quadrature signal channels at each array element.
Specifically, the in-phase (I) channel receives the signal
c*cos(.theta.), and the quadrature (Q) channel receives the signal
c*sin(.theta.), as noted above. In operation, the angle .theta. is
determined from the two input vectors (without regard to the
magnitude of the resultant vector sum), and then the magnitude of
the resultant vector is simply set (scaled) to c=e+s. Thus, there
is no need to actually "calculate" the value of the third (wasted
power) signal component. All that is required to determine the I
and Q input signals is the computation of the angle .theta. from
the two signal vectors e and s from which the signals
c*cos(.theta.) and c*sin(.theta.) can then be determined.
This configuration eliminates the need for high speed hardware
phase shifters at each array element. Each array element amplifier
is supplied an in-phase and quadrature component (two RF carrier
components that are in phase quadrature), and the magnitude and
sign of these two components are generated in the processor. Note
that the processor must be fast enough to perform these
calculations for each array element at the modulating signal rate.
This is well within modern processor/FPGA capabilities for the GPS
signals.
An implementation of a multi-beam, multi-signal system employing a
constant envelope composite signal in the context of a GPS
satellite transmitter generating spot beams and earth coverage
beams at the L1 and L2 frequency is shown in FIG. 5. In the GPS
satellite transmitter system shown in FIG. 5, the satellite is
required to transmit signals at both of the GPS frequencies L1 and
L2. Consequently, the antenna array includes a first array of
antenna elements that radiates at the L1 frequency and a second
array of antenna elements interspersed with the first array that
radiate at the L2 frequency. While the transmitter components for
only a single L1 antenna element and a single L2 are shown in FIG.
5, it will be understood that the system includes arrays of such
components respectively corresponding to the arrays of L1 and L2
antenna elements.
At the L1 carrier frequency, a composite in-phase (I) channel
digital signal is supplied to digital-to-analog converter 74, while
a composite quadrature (Q) channel digital signal is supplied to
digital-to-analog converter 76. The input I and Q channel digital
signals, which include both the spot beam signal and earth coverage
beam signals, are periodically generated by a processor at a rate
on the order of the modulating signal rate in accordance with
equations (1)-(5) above for each element in the array.
Specifically, the composite phase .theta. is periodically
determined from the amplitudes and instantaneous phases of the spot
beam and earth coverage beam signals, and digital signals having
amplitudes c*cos(.theta.) and c*sin(.theta.) are periodically
supplied to I and Q D/A converters 74 and 76, respectively. Note
that, for each antenna element, the instantaneous phase of the spot
beam signal is the sum of the instantaneous modulation phase of the
spot beam signal and the phase shift associated with that antenna
element required for beam steering. Likewise, for each antenna
element, the instantaneous phase of the earth coverage beam signal
is the sum of the instantaneous modulation phase of the earth
coverage beam signal and the phase shift associated with beam
forming for that antenna element. Thus, in general, the input I and
Q signals differ from element to element and are determined for
each antenna element in the antenna array at the digital signal
rate (i.e., at the signal modulation rate).
It will be understood that any of a variety of conventional
computing techniques can be used to determine the angle .theta.and
the I and Q input signals, including the use of lookup tables and
the like. As shown in FIG. 5, the processor for generating the
digital composite signals can be a special purpose processor such
as a field programmable gate array (FPGA) or an application
specific integrated circuit (ASIC). However, the processor of the
transmitter system is not limited to any particular hardware
configuration and could be implemented using, for example, a
general purpose processor executing a suitable program.
Referring again to FIG. 5, after D/A conversion, the analog I
channel signal is supplied to a signal modulator 78 (e.g., a mixer)
that modulates the in-phase L1 carrier signal (cos(.omega..sub.L1
t)) with the I channel signal. Similarly, the analog Q channel
signal is supplied to a signal modulator 80 that modulates the
quadrature L1 carrier signal (sin(.omega..sub.L1 t)) with the Q
channel signal. The modulated in-phase and quadrature signals are
combined by combiner 82 and amplified by high power amplifier 84
prior to transmission of the composite signal by the respective L1
antenna element. While not shown in FIG. 5 for convenience, the
transmitter hardware includes an array of similar D/A converters,
mixers, combiners and HPAs corresponding to the array of L1 antenna
elements.
Similarly, at the L2 carrier frequency, a composite in-phase (I)
channel digital signal is supplied to digital-to-analog converter
86, while a composite quadrature (Q) channel digital signal is
supplied to digital-to-analog converter 88. The analog I channel
signal is supplied to a signal modulator 90 (e.g., a mixer) that
modulates the in-phase L2 carrier signal (cos(.omega..sub.L2 t))
with the I channel signal. Similarly, the analog Q channel signal
is supplied to a signal modulator 92 that modulates the quadrature
L2 carrier signal (sin(.omega..sub.L2 t)) with the Q channel
signal. The modulated in-phase and quadrature signals are combined
by combiner 94 and amplified by high power amplifier 96 prior to
transmission of the composite signal by the respective L2 antenna
element. While not shown in FIG. 5 for convenience, the transmitter
hardware includes an array of similar D/A converters, mixers,
combiners and HPAs corresponding to the array of L2 antenna
elements.
Importantly, the combined signal supplied to each HPA 84 in the L1
array (and each HPA 96 in the L2 array) is a constant envelope
signal that includes both the spot beam signal and the earth
coverage beam signal. Consequently, each HPA 84 (and each HPA 96)
can be a saturated HPA that transmits a constant envelope signal.
The total transmitted power can be adjusted by controlling the
saturated power level of each HPA 84 via a power command, which
changes the amplifier bias point and allocation of the total power
among the various composite signal--beam combinations is effected
in the signal combining process. Note that, while each individual
antenna element generates a constant envelope signal, the power
levels of individual antenna elements may differ from each other,
depending on such factors as beam shaping and the number of beam
being supported by particular elements. For example, while most or
all of the antenna elements in the array would typically be used to
generate a steerable spot beam, only a certain number of the
antenna elements near the center of the array might be required to
generate a stationary earth coverage beam. Consequently, a group of
inner antenna elements in the array may be required to generate a
composite signal having a higher power level, while the remaining
outer elements that support only the spot beam may have lower power
levels.
While the configuration shown in FIG. 5 employs digital signals and
D/A converters to produce the composite phase modulated signals,
other hardware can be used to produce the composite signals. For
example, phase shifting could be accomplished with phase shifters,
as in FIG. 1; however, the phase shifters would be required to
change at the signal modulation rate, rather than at the much
slower rate at which the phase shifters of FIG. 1 change, which is
the rate at which beam pointing changes. This would require very
high-speed phase shifters which may be very expensive.
In accordance with yet another embodiment, another technique for
combining the various signal components is to utilize a time
division multiplexing (TDMA) scheme to time-interleave different
beams. For each antenna array element, the composite phase angle
for each beam can be calculated, as described above in connection
with FIG. 4. However, instead of summing the two vectors, the phase
angle that is applied to the array element is alternated between
the two angles. The interleaving of the beams can be accomplished
by generating digital signals with the appropriate phase at the
modulation rate of the signals. Accordingly, this embodiment can be
carried out using the same transmitter and antenna hardware shown
in FIG. 5.
Equally alternating between the two angles would effect equal power
distribution between the two beams, but any desired power ratio can
be achieved by generating more of one than the other. For example,
selecting the phase angle for beam A twice for every single
selection of the phase angle for beam B, would result in a power
ratio of 2.sup.2 (or 4). Equally alternating between A and B,
except that at every fifth time instant the angle for A is
generated twice in a row, would result in a power ratio of
(6/5).sup.2 (or 1.44). With this technique, virtually any power
ratio can be effected and the method can easily be extended to more
than two beams. There is some combining loss with this technique,
and the loss is dependent on the power ratios, but the power
efficiency is generally much greater than with linear combining
(i.e., the first embodiment described above), and this approach
eliminates the disadvantages associated with the "clipping"
technique (i.e., the second embodiment described above).
Note that the multi-beam, multi-signal concept is not dependent on
the particular signal combining technique that is used. Multiple,
independently steered beams, each with its own signal, can be
achieved with a single active phased array using any of the signal
combining techniques mentioned above, or others. With this
technology, additional beams and additional signals can be added
(or removed) with no hardware changes, provided sufficient total
power is available. In the context of satellite systems, signals
and beams can be added on-orbit via software changes and power can
be freely allocated among the various beams and signals.
The ability to transmit multiple beams, each with its own unique
signal, is an extremely desirable feature for a wide range of
diverse applications that require simultaneous transmission of
multiple signals, including future generations of GPS, GPS
augmentation systems, radar systems, wireless telephony, satellite
communication systems, the Global Multi-Mission Service Platform
(GMSP), systems employing code division multiple access (CDMA)
multiplexing and others communication systems.
The system of the present invention may be implemented using any of
a variety of hardware and software configurations and is not
limited to any particular configuration. For example, RF signal
amplification and/or phase shifting can be performed within an
integrated module containing the antenna element, or discrete
amplifier, phase shifter and antenna element components can be
employed. The size of antenna array not limited to any particular
number of radiating antenna elements and can be configured using
any appropriate number and arrangement of antenna elements required
to meet particular system requirements, such as beamwidth, scan
angle, antenna gain, and radiated power. Since beamwidth is a
function of antenna size, the smallest spot beam required will be a
determining factor in the overall size of the antenna array.
The multi-beam, multi-signal techniques of the present invention
can be implemented in a programmable waveform generator of a
transmitter system that is remotely reprogrammable. Such an
implementation allows remote programmability of transmission and/or
reception parameters (e.g., modulation characteristics) of units in
the field, such as satellites in orbit, communications
infrastructure, and mobile communication devices, including
wireless telephones to accommodate system requirements that change
over the lifetime of the communication equipment. The capability to
reprogram the waveform generator is especially valuable in the
space satellite context, where changes in required modulation can
take many years and a new satellite design to accomplish. With the
reprogrammable waveform generator of the present invention on
board, a satellite can be reprogrammed in orbit. In general, the
multi-beam, multi-signal techniques can be implemented using any
suitable combination of hardware and software.
Depending on beamwidth, antenna gain and power requirements, it may
not be necessary to use every antenna element in the array to form
every beam. For example, all antenna elements of the array may be
used to form a spot beam, while only a subset of those elements may
be needed to form an earth coverage beam. In this case, certain
antenna elements (inner elements) are transmitting composite
signals containing two signals (i.e., the spot beam signal and the
earth-coverage beam signal) combined in the manner described above,
while other antenna elements (outer elements) transmit only one
signal (i.e., the spot beam signal).
Although capable of transmitting a different signal with each
respective beam, the techniques of the present invention can be
used to send the same signals on more than one beam. For example, a
scenario could exist in which an earth-coverage beam carries a
first signal, while two separate spot beams oriented in different
directions both carry the same second signal. In general, any
combination of signals and beams can be generated by applying
appropriate phase shifts to the individual antenna elements.
Having described preferred embodiments of new and improved methods
and apparatus for multi-beam, multi-signal transmission for active
phased array antenna, it is believed that other modifications,
variations and changes will be suggested to those skilled in the
art in view of the teachings set forth herein. It is therefore to
be understood that all such variations, modifications and changes
are believed to fall within the scope of the present invention as
defined by the appended claims. Although specific terms are
employed herein, they are used in a generic and descriptive sense
only and not for purposes of limitation.
* * * * *